Monoamine oxidase (MAO) is a key enzyme responsible for biogenic amine catabolism. There are two forms of MAO (A and B) which differ in pharmacological properties, developmental regulation and hormonal responsiveness. Somatic cell genetic studies suggest that genes for both enzymes are present on the human X chromosome, which makes the MAO A-MAO B system a relatively unusual isozyme pair whose genes are linked but expressed in different sets of neurons. Although immunocytochemical data suggest that the form of MAO expressed by an aminergic neuron may be determined by its neurotransmitter phenotype, little is known about the respective physiological roles of MAO A and B, their structural or genetic relationship, or the molecular mechanisms which govern their independent regulation. Our working model is that (1) MAO A and B are homopolymers with different but homologous primary and secondary structures and (2) are coded by genes which are tightly- linked on the X chromosome. (3) MAO A and B genes arose by duplication and aquired disparate regulatory sequences as the enzymes evolved to fulfill specialized roles in different cells and tissues. Complementary cDNA clones will be isolated for human and rat MAO A and B using a combination of immunological and synthetic oligonucleotide screening approaches. The cDNA clones will be tested for MAO coding sequences by comparison of nucleotide sequence with fragments of amino acid sequence, mRNA size and cell type-specificity of expression, and translation of hybrid- selected mRNA. The cDNAs will be used to (1) map the genes for the enzymes, (2) examine mRNA levels in tissues and cultured human fibroblasts, rat sympathetic neurons and rat pheochromocytoma cells, (3) express the human cDNAs in mouse cells to test whether each enzyme has a single subunit type, and (4) isolate genomic clones for human MAO A and B to compare the intron-exon structure of the genes and putative, linked regulatory sequences likely to be responsible for their independent developmental, hormonal and cell type-specific regulation. The studies proposed should resolve a number of long-standing questions about the structure and genetics of MAO, make possible new strategies for exploring the molecular mechanisms of their regulation and their potential role in human disease, and provide a new model system for investigating aspects of aminergic neurotransmitter phenotype in neurons.